The invention relates to a process for preparing crimped synthetic fibers by the compressive crimping process, in particular for acrylic fibers, and to an apparatus for carrying out the process. In particular, the invention relates to a process for continuous compressive crimping during a continuously running fiber spinning and aftertreatment process involving high tow weights above 100,000 dtex and production speeds above 200 m/min.
Processes and apparatuses for crimping synthetic fibers are known. With the most preferred form of compressive crimping, the tow is passed through two guide rolls into a crimping box where the tow accumulates and is kept back under pressure, the tow becoming folded in small zigzags to form the so-called crimp. Three of the four walls of the crimping box are fixed, while the fourth wall is formed by a movable plate on which pressure can be imposed. When the internal pressure of the crimped tow has become equal to the pressure which prevails on said movable plate, the plate is forced upward and the compressed tow leaves the box through the slot formed in this way.
It has now been found that the previously disclosed processes and apparatuses of this kind have the disadvantage, in particular in the crimping of acrylic tows, that they can only be used for crimping tows at production speeds up to about 150-200 m/min. At higher speeds, above about 200 m/min, acrylic tows tend to become compacted. The reason for that is that, at high speeds and high tow weights as occur chiefly in continuous spinning and aftertreatment processes as described, for example, in EP-A No. 98,477, large amounts of fiber accumulate in the stufferbox within a very short time and their pent-up kinetic energy must be dissipated to avoid compacting. There has been no shortage of attempts to take account of this fact, for example by cooling the intake rolls, by guiding the crimped tow in a specific way (DE-A No. 1,435,438) or by wetting the tow with moisture (U.S. Pat. No. 3,041,705). By cooling and specific tow guidance in the stufferbox alone, however, it is impossible to achieve high production speeds as occur in continuous spinning and aftertreatment processes. In addition, the compressive crimping of moist acrylic tows has the disadvantage that the crimp is very unstable and frequently leads to so-called hack points during compressive crimping. Hack points are to be understood as meaning crimping damage in the tow which leads to holes in the crimped filament assembly and gives rise to staple length shortening and short fibers.